GeomP1 3.1­3.2 ­ Angles and Angle Measure
Don't Blink! The Angles are Coming!
Don't be Obtuse!
GeomP1 3.1­3.2 ­ Angles and Angle Measure
That's Acute Angle
No need to be shy.
That's right! Right angle!
GeomP1 3.1­3.2 ­ Angles and Angle Measure
An angle is formed by two rays (or sides) with the same endpoint, or vertex.
A straight angle is when two rays are part of the same line and have the same vertex.
S
C
T
Two rays are opposite rays if they form a straight angle. What are the opposite rays above?
Naming Angles
Safest: use three points
A
C
Easy: use the vertex
Depending on the diagram: use a number
B
GeomP1 3.1­3.2 ­ Angles and Angle Measure
Naming Angles
Safest: use three points
C
Easy: use the vertex
Depending on the diagram: use a number
Naming Angles
Safest: use three points
1
Easy: use the vertex
Depending on the diagram: use a number
GeomP1 3.1­3.2 ­ Angles and Angle Measure
Why don't we just use the vertex?
What does LX mean?
X
Why don't we just use the vertex?
How many LX's are there?
B
C
D
A
X
GeomP1 3.1­3.2 ­ Angles and Angle Measure
Interior
Exterior
Measuring Angles
Angles are measured in degrees.
A straight angle has a measure of 180o.
A right angle has a measure of 90o.
(There are two right angles in a straight angle). GeomP1 3.1­3.2 ­ Angles and Angle Measure
Types of Angles
Acute Angle
Obtuse Angle
Between 0° and 90°
Between 90° and 180°
Right Angle
Straight Angle
Exactly 90°
Exactly 180°
Measuring Angles with a Protractor
1.
2.
3.
4.
Put the center of the protractor at the vertex.
Always start with one side of the angle at 0°.
Count up from 0° until you reach the second side.
Read the angle measure.
80
90
100
70
110
60
110
50
100
90
120
40
150
70
130
60
140
50
130
140
30
120
80
0°
40
150
30
160
20
160
10
0
20
170
10
180
0
170
180